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Answer: Extension members let you add methods to an existing type without subclassing, recompiling or modifying it. The classic form is a static method whose first parameter uses this; newer C# adds extension blocks. An extension block is a block in a non-nested, nongeneric, static class that contains extension members for a type or an instance of that type. (S1)

Classic extension method

public static class StringExtensions {
  public static int WordCount(this string s) =>
    s.Split(' ', StringSplitOptions.RemoveEmptyEntries).Length;
}

"Hello Extension Methods".WordCount(); // 3

Extension blocks

public static class NumericSequences {
  extension(IEnumerable<int> seq) {
    public IEnumerable<int> AddEach(int n) => seq.Select(x => x + n);
  }
}

Extension blocks can be generic, and individual members can add their own type parameters.

Rules

• The extension must be in scope (using its namespace).
• An instance method on the type always wins over an extension with the same signature.
• Extensions cannot access private members.

Claims

• id: C1 claim: Extension members add methods to existing types without modifying them. source: S1 evidence: "methods to existing types without creating a new derived type"
• id: C2 claim: Extensions must be inside a non-nested, nongeneric static class. source: S1 evidence: "Both forms of extensions must be defined inside a non-nested, nongeneric static class."
• id: C3 claim: The docs show extension members for numeric sequences. source: S2 evidence: "/// Contains extension members for numeric sequences."
• id: C4 claim: Extension blocks can be generic. source: S2 evidence: "Generic extension blocks"

Sources

• [S1] Extension members - C#. Microsoft Learn (.NET docs). https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.
• [S2] Extension member declarations - C# reference. Microsoft Learn (.NET docs). https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/extension . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_k7a6rrwq,
  author = {{iinventedlight}},
  title = {{C\# extension methods and extension blocks}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/csharp-extension-methods/},
  note = {Permanent ID iil:note:k7a6rrwq. License: CC-BY-4.0.},
}

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